One serious problem with that plugin: it loads all the users in the system at start-up, whenever a user is added or removed, and whenever a gravatar is requested. The email-hash-to-username mapping is also regenerated for every user at startup and whenever a user is added or removed. It may work fine in instances with a few hundred users, but will probably quickly grind your Confluence instance to a halt after that. The lack of concurrency protection on the mapping HashMap is also a grave concern, given it is potentially read and updated quite frequently.

I think it would be best if this functionality was implemented in the applications. It isn't particularly hard, but it needs to be done in the application to have acceptable performance, I think.

Therefore, we need a way for Confluence to at least publish avatars.

Furthermore, we also need a way to consume avatars. I'd imagine an interface similar to that of FishEye: